[lit] Fix undefined symbol ArgumentError

Summary:
`ArgumentError` is not defined by the Python standard library.
Executing this line of code would throw a exception, but not the
intended one. It would throw a `NameError` exception, since `ArgumentError`
is undefined.

Use `ValueError` instead, which is defined by the Python standard
library.

Reviewers: echristo, delcypher, beanz, ddunbar

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D25410

llvm-svn: 283708